This invention relates to induction motors. More particularly, the invention relates to a method and apparatus to maximize the efficiency of an induction motor.
As an AC induction motor rotates, the magnetic fields of the rotor and the stator interact. The stator windings are typically connected to a supply in three-phase form or single phase form. By applying a voltage across the windings, a radial, rotating magnetic field is formed. The rotor has solid aluminum bars cast in a xe2x80x9csquirrel-cagexe2x80x9d configuration. The rotating magnetic fields produced by the stator produce a current in the aluminum bars of the rotor. This produces a magnetic field in the aluminum bars which interacts with the rotating magnetic field of the stator to generate torque on the rotor. The rotor reacts to the magnetic field, but does not travel at the same speed. The rotor actually lags behind the speed of the rotating magnetic field. This lag is called slip, and is essentially a comparison of the speed of the rotor and the speed of the magnetic field. The slip typically increases proportionately with increases in load.
Induction motors run less efficiently when lightly loaded. In order to increase efficiency of the motor, the flux of the motor may be reduced by utilizing the flexibility built into most variable speed drives. However, determining the ideal flux for maximum efficiency often requires the use of expensive sensors.
A common approach used to increase the efficiency of the induction motor is to sense the difference between the respective phases of the energizing voltage and current at the motor terminals. This requires identifying the zero crossings of the voltage and current waveforms. However, when the motor voltage is pulse-width modulated at a low frequency, phase detection is difficult because the current has some of the same components as the waveform of the carrier frequency. Thus, identifying the zero crossings of the phase current may be difficult.
Another approach used to increase the efficiency of an induction motor is to control the slip of the motor for maximum efficiency. To measure the slip, the actual rotor speed must either be measured or estimated. However, measuring or estimating rotor speed is very expensive.
Accordingly, the apparatus and method of the invention provide an accurate estimate of the slip of the motor using only one phase current sensor combined with knowledge of the voltage and current at the motor. In the apparatus of the invention, the motor includes a monitoring circuit to monitor an analog DC bus voltage and an analog DC bus current. A circuit connected to the monitoring circuit estimates the predetermined slip of the motor. A compensating circuit connected to the circuit adjusts the voltage applied to the motor such that the motor operates at the predetermined slip.
In the method of the invention, three phase AC power is supplied to energize the motor. A DC bus voltage and a DC bus current are measured. An actual torque producing current value is calculated based on the DC bus voltage and the DC bus current along with an estimated phase voltage. The actual torque producing current value and the estimated torque producing current value are compared. If the actual and estimated torque producing current values are different, an error term is produced representing that difference. The estimated phase voltage value is then changed based upon the previous estimated phase voltage value and the error term. The three phase AC power supplied to the motor is adjusted based on the estimated phase voltage.
The principal advantage of the invention is to optimize the running efficiency of the motor by determining the motor slip.
Another advantage of the invention is to provide an accurate estimate of the slip of the motor using only one phase current sensor.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.